Physician Amneet Sandhu of the University of Colorado Denver and colleagues analyzed more than 42,000 admissions for heart attack at Michigan hospitals that occurred from 2010 through 2013. He reported March 29 at a meeting of the American College of Cardiology that his team found a 24 percent increase in heart attack admissions on the Monday after the spring-forward clock change to daylight saving time when compared with other Mondays throughout the year. In contrast, the researchers detected a 21 percent decrease in heart attacks on the Tuesday following the switch back to standard time in the fall, shortly after getting an extra hour. Heart attack rates on other days during those weeks were not substantially different from average. The results bolster previous smaller studies that linked daylight saving time to heart attacks.

The spikes translated into eight more heart attack patients at Michigan hospitals on the spring Monday and seven fewer on the fall Tuesday, Sandhu said. Because the hour gained or lost comes during the night, the finding raises questions about the risk of disrupting the sleep cycle. “This might precipitate heart attacks in particularly vulnerable patients,” he said. Emergency rooms should be aware of the pattern, he added.